For a variety of applications there is a requirement for a drive circuit able to provide regulated drive of a load from some dc power supply. One such drive circuit in accordance with the prior art is shown in FIG. 1 and incorporates a Zetex Semiconductors plc. voltage mode boost converter ZXSC410. The circuit may, for example, be connected to a battery power supply and is able to provide a regulated output voltage Vout to drive an attached load. Applications of the circuit include the provision of system power for battery-operated portable products, the biasing of LCDs, and local voltage conversion. Further details of the converter device are shown in FIG. 2. Operation of the converter and drive circuit are as follows. The converter includes a temperature compensated bandgap reference circuit, connected to receive the supply voltage Vcc, and from which are derived all threshold voltages and internal currents. A dynamic drive circuit receives an input signal from a monostable circuit, and depending on that input signal the output of the dynamic drive is either “low” or “high”. In the high state a current source drives the base of the external transistor Q1. The device includes a switching circuit comprising two comparators Comp1 and Comp2, a gate U1, a monostable and the drive output. Normally the drive output is “high” and the external switching transistor Q1 is turned on. Current then ramps up in the inductor L1, the switching transistor Q1, and the external current sensing resistor R1. The voltage (across sensing resistor R1) is sensed by comparator, Comp2, at input SENSE. Once this exceeds a predetermined threshold, comparator Comp2, through gate U1, triggers the re-triggerable monostable and turns off the output drive stage for a predetermined time (typically 2 microseconds). While the switching transistor is off, the inductor discharges to the load causing the output voltage VOUT to rise. After the predetermined time, if VOUT is below the nominal value, a new charge cycle begins, thus ramping up the output voltage. When the output voltage reaches the nominal value and the voltage feedback pin VFB receives an input voltage greater than a predetermined threshold, the monostable is forced “on” from Comp1 through gate U1. This stalls operation until the feedback voltage falls below the relevant threshold again. The above action then continues, to maintain regulation.
Thus, the above drive circuit can be described as operating in a skip mode (in other words, a stop-start mode), as the switching transistor is switched between off and on, being switched off according to fixed, predetermined threshold voltages, and remaining off for a fixed, predetermined time, before switching back on.
For various applications the above drive circuit provides excellent regulation, for example providing an output voltage that changes by less than 1% in response to a change in supply voltage over the full supply range from lithium-ion cells. The circuit may also be used to provide a regulated output from supplies in the range 1.65V to 8V.
However, a disadvantage with such known circuits operating in skip (start-stop) mode is that they can produce relatively large ripple, with the resultant emission of electromagnetic radiation, and furthermore the frequency of the skipping can be within the audio range, resulting in the coil core emitting an audible whistle, which in many applications (such as the driving of LEDs in a torch) is undesirable. The reason for the audible whistling is that the prior art has an output power which is fixed by outside parameters, such as supply voltage, peak inductor current, inductor value, etc. and the fixed 2 microseconds off time referred to above. If this power is excessive, the only way the device can control it is to stop working completely until the output voltage or current is within the desired range. The circuit will perform several cycles at an inaudibly high frequency, then stop for a period which can be quite long. This slow on and off cycling results in an audible whistle from the inductor.